How far can you kill? Things like ants, spiders, and other bugs, we can squish without much thought, right? But if it's a fish, we feel a little reluctant when killing it. We begin to feel pressures of taking a life. It gets worse if the fish is replaced by an amphibian, and even more so if the amphibian becomes a reptile. The pressure is worse if the reptile becomes a bird, and even worse when the bird becomes a mammal. The higher up the chain we move towards the more advanced creatures, the more we hate to kill them. And eventually we hit a hurdle too high to overcome. We're unable to kill anything beyond that point. I think the highest hurdle would be... people.